The invention relates to a laser cutting method for cutting carbon-manganese (C—Mn) steel using a laser source of the ytterbium-based fiber type.
At the present time, laser cutting using a laser source of the CO2 type to generate a laser beam, with a wavelength of 10.6 μm and a power ranging up to 6 kW, is widely used in industry.
This method is used in particular for cutting C—Mn steels. Within the context of the invention, the term “C—Mn steel” is understood to mean any non-alloy steel or low-alloy steel, the carbon and manganese contents of which are less than 2% by weight and the contents of the other alloying elements optionally present are less than 5% by weight.
However, the cutting speeds that can be achieved and the cutting quality that results therefrom are very variable, depending on the material to be cut and, moreover, depending on the cutting method parameters adopted, such as the nature of the assistance gas, the diameter of the focused beam, the power of the incident laser, etc.
Thus, CO2 lasers cannot be used with assistance gases of low-ionization potential, for example such as argon, without the risk of generating parasitic plasmas that could impair the method.
Furthermore, CO2 lasers are limited in terms of power, thereby directly impacting the cutting speed.
In addition, the fact of having to guide the laser beam from the laser generator right to the focusing head, that is to say the cutting head, has drawbacks, especially as regards alignment of the optics in the optical path. This is because guiding optics are generally polished and/or coated copper mirrors and their positions determine the path followed by the laser beam.
Therefore, the alignment of the mirrors must be perfect in order to ensure optimum entry of the laser beam into the focusing head or cutting head. Now, the position of these mirrors is generally adjusted by mechanical means, which may easily go out of alignment according to the wear of parts and the environmental conditions, such as the ambient temperature, moisture content, etc.
In addition, the optical path of the beam must necessarily be kept in an inert atmosphere in order to avoid any contamination and to maintain a medium with a constant optical index, which is necessary for good propagation of the beam. These conditions make it possible for the properties relating to the beam diameter and the transverse distribution of the beam energy, and also the beam quality properties, to remain satisfactory for the method, the quality factor for beam parameter product (BPP) of the high-power CO2 laser beams used in cutting generally being between 3 mm.mrad and 6 mm.mrad. Such an atmosphere also makes it possible to preserve the guiding optics and to prevent them from deteriorating.
However, this is not practical in an industrial situation, as it complicates the installation and incurs additional costs.
In an attempt to alleviate these problems, it has been proposed to carry out the cutting with a laser device of the Nd:YAG type within which the beam is generated by a resonator containing a solid amplifying medium, namely a neodymium(Nd)-doped YAG rod, and then sent via an optical fiber to the focusing head.
However, this solution is not satisfactory from the industrial standpoint either.
More precisely, it has been found that cutting with a laser beam output by an Nd:YAG laser source with a wavelength length of 1.06 μm gives poor results in terms of cutting quality and cutting speed, in particular when cutting a workpiece made of C—Mn steel.
This is because Nd:YAG lasers have quality factors (BPP values) unsuitable for the laser cutting process hence their range from around 15 mm.mrad to 30 mm.mrad, depending on the laser source.
Now, it should be understood that the higher the quality factor of a laser, i.e. the higher the product of the focused beam waist multiplied by the beam divergence, the less effective the laser beam for the laser cutting process.
In addition, the transverse energy distribution in a focused Nd:YAG laser beam is not Gaussian but has a top-hat profile, while beyond the focal point the transverse energy distribution is random.
The limits on using Nd:YAG lasers in laser cutting, in particular for C—Mn steel, are therefore immediately understood.
More generally, to cut a C—Mn workpiece by laser cutting with an Nd:YAG laser is far from being obvious when it is desired to achieve cutting speeds and cutting qualities that are acceptable from the industrial standpoint.
The problem that arises is therefore how to provide an improved and industrially acceptable method for cutting C—Mn steels with a laser beam, which can achieve, depending on the thickness in question, speeds ranging up to 15 to 20 m/min, or even higher, and good cutting quality, that is to say straight cutting faces, no burrs, limited roughness, etc.
The solution provided by the invention is therefore a laser cutting method for cutting a C—Mn steel workpiece, characterized in that laser beam generation means comprising at least one ytterbium-containing fiber for generating a laser beam used to melt the workpiece and thereby perform the actual cutting, and in that the quality factor of the laser beam is between 0.33 and 8 mm.mrad.
The laser beam generation means comprise an exciter, preferably several exciters, which cooperate with at least one excited element, also called amplifying medium, in order to generate the laser beam. The exciters are preferably several laser diodes, while the excited elements are fibers, preferably silica fibers with an ytterbium-doped core.
Furthermore, for the purpose of the invention, the terms “laser beam generation means” and “resonator” will be used indiscriminately.
Depending on the case, the method of the invention may include one or more of the following features:                the fiber(s) is(are) formed from an ytterbium-doped core clad with silica;        the laser beam generated by the ytterbium-based fiber has a wavelength between 1 and 5 μm, preferably between 1.04 and 3 μm;        the laser beam generated by the ytterbium-based fiber has a wavelength between 1.07 and 1.1 μm, preferably of 1.07 μm;        the laser beam has a power of between 0.1 and 25 kW, preferably between 0.5 and 15 kW;        the laser beam is a continuous or pulsed laser beam, preferably a continuous laser beam;        the workpiece to be cut has a thickness between 0.25 and 30 mm, preferably between 0.40 and 20 mm;        the cutting speed is between 0.1 and 20 m/min, preferably from 1 to 15 m/min;        the assistance gas for the laser beam is chosen from nitrogen, helium, argon, oxygen, CO2 and mixtures thereof, and, optionally, it further contains one or more additional compounds chosen from H2 and CH4;        the quality factor of the laser beam is between 1 and 8 mm.mrad, preferably greater than 2 mm.mrad, even more preferably greater than 3 mm.mrad and/or preferably less than 7 mm.mrad and even more preferably less than 5 mm.mrad;        the cutting speed for a steel workpiece with a thickness between 0.4 mm and 3 mm, using oxygen as assistance gas at a pressure of between 0.2 and 6 bar, is between 6 and 15 m/min;        the cutting speed for a steel workpiece with a thickness of between 3 mm and 6 mm, using oxygen as assistance gas at a pressure of between 0.2 and 6 bar, is between 2 and 6 m/min;        the cutting speed for a steel workpiece with a thickness of between 6 mm and 10 mm, using oxygen as assistance gas at a pressure of between 0.2 and 6 bar, is between 1 and 3 m/min;        the cutting speed for a steel workpiece with a thickness of between 10 mm and 20 mm, using oxygen as assistance gas at a pressure of between 0.2 and 6 bar, is between 0.1 and 2 m/min;        more generally, the assistance gas pressure is between about 0.1 bar and 10 bar, and is chosen according to the thickness that is to be cut; and        the diameter of the gas injection orifice is between 0.5 and 5 mm, typically between 1 and 3 mm.        